If you're into Clevelands and it's a race
style head you're after, then things are rosy. But if you want
something for the street it's a different matter. There isn't
much around and John Konstandjnou is pretty sure that there are
enough frustrated Cleveland owners out there to take a few of
these new alloy Cleveland heads off his hands.
Long-time friend and associate, Con from
P.R.E. (Performance Race Engines), has been in on the
development from the beginning and it's his dyno you're looking
at in the shots. The idea here was to do a back to back test of
the new units compared to a pair of heavily worked factory iron
units. This was the first time the new design had been run and
we were there waiting.
Flow figures, and John and Con's
experience, indicated that the design would be good for
something more than five hundred horsepower. But flow figures
aren't dyno figures so you can't be sure of what you've got
until you switch it on. But as the graph shows, the numbers were
pretty much where the guys figured they'd be. Having things work
out like this is good reward for the hard work involved in
bringing something like this from concept to market.
The hard work consisted of getting a
factory iron head, sticking it on the flow-bench, finding all
the problems and fixing them. The first limitation with factory
heads is that they're too big in the portss, particularly near
the gasket faces. As we've described in previous articles about
flow in different situations, a smaller port volume for a given
flow results in faster airspeed and more efficient breathing.
You can see where the iron heads have been filled and re-shaped
in the shots.
The large 'funnels' at both gasket faces
are gone and things are different further into the ports too. In
the inlet, for instance, there was a dead spot on the cylinder
side which had to be filled out while the wall on the exhaust
side had to be opened out correspondingly. The short-turn radius
was also modified. It's not only speed that matters, direction
is also important and that brings us to the chambers. You'll
notice that both sides of them have been filled, bringing them
in closer to the centre. The peak opposite the spark plug is
there to direct air down into the cylinder and prevent it
shooting across the chamber and out the exhaust valve on
overlap. It also creates swirl which is a great help in getting
the charge full burnt.
Con explained that good swirl is vital,
particularly in current generation motors. This is because the
lower compression engines have larger clearance volumes and
don't generate as much squish. Less squish means less turbulence
and therefore, less efficient combustion. Increasing swirl
compensates for reduced squish. However, a 351 (or stroker)
Cleveland with a filled-in chamber doesn't have a problem with
lack of squish. In fact, the increased squish area created by
the extra material in the chambers, combined with the shape of
that material, has created a chamber with both squish and swirl
for maximum turbulence. So, no matter how you build your engine,
you'll be able to find a way to keep the charge moving quickly
enough to get it all burnt at high engine speeds.
The most effective cure for Cleveland
exhaust ports is to cut off the outer sections of the factory
ports, bolt a large bar of aluminium to the exposed area, and
carve new ports into the alloy that meet what's left of the
original versions. This is high-porting but it's expensive and
requires custom headers which adds even more to the cost. The
next best solution is to fit steel plates that effectively raise
the floors of the ports and prevent flow-loss by keeping the
volume more consistent out to the exhaust flange. This improves
things considerably and port-plates were fitted to the iron
heads on the motor shown. However, shaping the ports in the new
head very carefully has removed the need for the cures just
outlined. The floors in the new ports are higher and the roofs
are also couple of millimetres higher than the factory line,
but, importantly, the new port shapes are still compatible with
standard headers.
Plug position is important and in the new
heads it's closer to the centre of the chamber. Having it in
this position creates better flame travel and allows a touch
more timing and compression before detonation occurs. A
centrally placed plug also helps get a better burn with a
lumpy-topped piston because the flame can travel down either
side of the dome more easily. The angle of the plugs is also a
bit better in the new heads.
On the matter of detonation, there are a
couple of other design elements that are important. The
thickness of metal across the roof of the combustion chamber is
one and it should be as even as possible to avoid hot spots.
Also, coolant flow through the heads should be generous and
free. Some people think this is less important in aluminium
heads because of aluminium's greater capacity to conduct heat,
however John and Con disagree and have paid a great deal of
attention to the design of the coolant jacket.
The idea of the back to back test that we
photographed was to compare heads with similar flow
characteristics. Getting the iron heads to match the new design
took quite a bit of work. By the time the cfm figures were the
same, the factory head had an inlet port volume of 235cc
compared to the 215cc measured in the new units. The same valve
gear, extractors, intake manifold, compression, and modified
Holley 750 were retained although the jets two sizes smaller
were needed front and rear in the carburettor. Both sets of
heads peaked at about 525hp but you can see in the graph that
the alloy heads were higher for longer virtually every step of
the way with the greatest differences showing up between 5000rpm
and 6000rpm.
As we go to press, this is the only hot
test we've seen for these heads. There are other engines being
screwed together as we speak but the numbers from those won't be
available until after deadline. Getting a particular power
rating out of a motor can be done in a number of ways but we
asked Con for a few general recommendations. He pointed out that
two popular cams for street Clevelands are the SVO Motorsport
with a duration of 248º @0.050" or the Crane unit 246º
@0.050". Each of these cams would make perhaps 450-500hp
with good torque at 10.5:1 compression burning Optimax. This
set-up wouldn't need a basketful of expensive components either.
A quality single-plain manifold, like the
TFC unit in the shots, is the starting point for the next step.
Combined with a 0.580" - 0.600" flat-tappet cam
measuring 250º @0.050" on the inlets and 260º
@0.050" on the exhausts, and maybe 106º lobe separation,
would make between 500-550hp with 11:1 compression. The next
step would be a bit of porting, 12:1 compression and suitable
cam. This combination would probably show around 600-650hp
without difficulty. Moving to a roller cam with internals that
spin to 8000rpm would see figures in the range of 650hp or
700hp.
The heads will be available under the name
CHI and you can find out more by calling John on 0419 245 265
or Con at P.R.E. on 03 9357 7668.
|
